In an optical storage system, data on an optical disk is stored in spiral or concentric tracks. A laser beam is directed through a series of optical elements and focused as a beam spot on a surface of the disk. Typically, a coarse carriage, on which is mounted an optical head with an objective lens through which the beam passes, is moved along a radial path to enable the beam spot to be moved between the inner and the outer tracks of the disk near the inner and outer diameters of the disk. The resolution of the coarse carriage and controlling servo loop is generally sufficient only to permit the beam spot to be positioned within a few tracks (such as .+-.5 tracks) of a desired target track. Consequently, a fine tracking actuator is employed to supplement the coarse carriage by finely controlling the beam spot to position and maintain it on a single target track.
In order to achieve accurate and rapid power-on calibration as well as track seeking, the position of the coarse carriage should be known. A coarse position sensor associated with the coarse carriage generates a coarse position sensing (CPS) signal, which bears a known relationship to the position of the coarse carriage relative to the drive frame, to provide the drive controller with this information.
As will be appreciated, it is desirable that the CPS signal be substantially linear over the range of motion of the coarse carriage and be substantially independent of variations due to such conditions as environment or component heating and aging. It is also desirable for a sensor to be small, inexpensive and of low mass so as to have as little impact as possible on the seeking and tracking performance of the drive.